1. Field of the Invention
The present invention relates to a variable optical attenuator used in the field of optical communication.
2. Description of the Related Art
As the optical communication technology advances, the optical communication is conducted through a longer optical path and a more complex line. An optical line taking various paths is required to be connected to an optical communication apparatus and an optical transmission apparatus to process the optical signal. The intensity of light (referred to below as the “amount of light”) varies according to the path taken by the line. If the lines which differ from each other in the amount of light are connected together, the deterioration of the S/N ratio may occur to reduce the optical transmission quality. In order to stabilize the transmission quality, there is needed an optical attenuator which substantially reduces the amount of light of one line having a larger amount of light to that of the other lines. Since the amount of light of a line often changes, it is essential to make the attenuation adjustable.
A variable optical attenuator is disclosed in various patent documents. For example, in a variable optical attenuator using a Faraday rotor based on the magneto-optics effect, which is disclosed in Japanese Laid-Open Patent JP 2001-249313 A, a Faraday rotor, a polarizer and an analyzer are arranged between the input optical fiber and the output optical fiber to align the optical axis. Whenever the light propagates in each of these elements, an insertion loss occurs. In addition, due to the polarization, the optical axis is deflected to make it difficult to align the optical axis between the input side and the output side.
Japanese Laid-Open Patent JP 2000-131626 A discloses a variable optical attenuator which changes the optical coupling by use of a reflecting mirror. In the variable optical attenuator, disposed between the input optical fiber and output optical fiber are a lens and a reflecting mirror positioned at the focal point of the lens. The angle of the reflecting mirror is changed to reflect the optical axis so that the amount of light propagating from the input optical fiber to the output optical fiber is changed. Consequently, many elements including the input optical fiber, the output optical fiber, the lens and the reflecting mirror positioned at the focal point of the lens are required to be accurately assembled; the configuration of the elements is complex to make the assembly more difficult.
Japanese Laid-Open Patent JP 2002-107639 A discloses a variable optical attenuator which adjusts the optical shielding on the optical path. In the optical attenuator, the input optical fiber and output optical fiber are disposed opposite each other with the axis of each optical fiber oriented to the identical direction; a shielding member is disposed on the optical path between the input optical fiber and output optical fiber. The movement of the shielding member allows the shielding of the optical path to be changed, thereby changing the amount of light. The optical axis alignment of the optical fibers positioned opposite each other is needed. In addition, the initial loss may change according to the variation of the distance between the two fibers, reducing the variable range accordingly. Furthermore, the variation among the attenuators is readily introduced.
A variable optical attenuator in which a linear optical fiber is partially deformed is disclosed in PCT International Publication WO 00/16143. In the optical attenuator, an optical fiber formed by removing the cladding thereof to expose the glass part so as to increase the attenuation is locally deformed. This causes the local concentration of the stress in the optical fiber to make it extremely fragile. In addition, a large deformation of the optical fiber may break the optical fiber and thus it is not possible to deform the optical fiber to a large extent, so that a large attenuation is difficult to achieve. Disclosed in Japanese Laid-Open Patent JP 6-18793 A is a variable optical attenuator formed by winding an optical fiber around a circular supporting member and partially deforming the optical fiber by use of a U-shaped plunger. In the optical attenuator, the optical fiber may undergo the local concentration of the stress. A variable optical attenuator in which an optical fiber is wound around a cylindrical core member to achieve attenuation is disclosed in Japanese Laid-Open Patent JP 10-20123 A. The optical attenuator is simple in structure, but requires the provision of the core members which differ in diameter according to the desired attenuation and thus time-variant amount of light is difficult to cope with. Disclosed in Japanese Laid-Open Patent JP 2001-228337 A is a variable optical attenuator which changes the diameter of a circularly wound optical fiber.
In most of the conventional variable optical attenuators, some optical elements are interposed between the input fiber and output fiber. Therefore, the optical path coupling between the input fiber and output fiber is indispensable, and thus the optical axis alignment is required. Since the core of an optical fiber is about 10 μm in diameter, even a slight difference of the optical axis causes an optical loss due to the misalignment of the optical axis. In addition, the optical elements including a Faraday rotor, a mirror and a lens, interposed between the input fiber and output fiber, have the optical loss of their own. In the case where the overall optical loss including the loss resulting from the misalignment of the optical axis and the loss of the optical elements is larger than the amount of light to be adjusted, not an optical attenuation but an optical amplification is needed. Therefore, it is essential to minimize the initial optical loss (attenuation) caused by the optical attenuator itself. The reduction of the initial optical loss of the optical attenuator requires the use of low-loss elements as well as the correspondence of the optical axis of the input fiber with that of the output fiber, and thus the optical attenuator becomes expensive. For the purpose of minimizing the initial optical loss, a proposal has been made that a fused bi-conical silica glass coupler is employed and the free end of the optical coupler is deflected, as disclosed in PCT International Publication WO 00/16143. In this case, however, the increasing of the attenuation of the amount of light requires a large amount of deflection. A large deflection of the fragile material of glass causes the local concentration of the stress, whereby there arises a risk that the material becomes more fragile due to the repeated deflections over a long period of use.